Dehydration:

We saw the hydration of an alkene under acid catalyzed conditions, gives an alcohol. Now, we will study the reverse of this reaction, the elimination of an alcohol to make an alkene and water. Both reactions are acid catalyzed.

Notice that this reaction is reversible. We must take advantage of Le Chatelier's Law of Mass Action to push the reaction to the right or pull the reaction to the left: In order to push the reaction to the right, we must add reactants or take away products. To pull the reaction to the left, we must add products or take away reactants.

To encourage hydration, we add a huge excess of H₂O and/or try to remove the alcohol. To encourage dehydration, we use dry acid and/or remove the alkene if possible.